1. Field of the Invention
The present invention relates generally to video tape recorders and more particularly to a recording apparatus in which the chrominance portion of a standard color-television signal is encoded for recording as part of the luminance portion of the standard color television signal.
2. Description of the Prior Art
The present United States color television standard is the result of efforts by the National Television System Committee, NTSC, during the infancy of the television to develop a standard color-television system compatible with the then existing black and white system. In the black and white system a signal is developed, as by a black and white television camera converting the luminosity of successive points along successive horizontal scan lines of an image into corresponding signal levels. Blanking and synchronizing information, including horizontal-synchronization pulses, are combined with the image signal to develop a monochrome, or black and white, television signal. The monochrome signal may then be transmitted as an amplitude modulation (vestigal sideband) of a visual-carrier signal located 1.25 megahertz above the lower boundary of a 6 megahertz television channel. Aural information, in the form of an audio signal, is transmitted with the monochrome signal as frequency modulation of an aural-carrier signal located 4.5 megahertz above the visual carrier.
In the NTSC standard color television system, three signals designated R, B and G, which represent the red, blue and green portions, respectively, of an image, are developed, as by a color-television camera. By combining the R, B and G signals so as to develop a signal which is 30% R signal, 11% B signal and 59% G signal and combining therewith blanking and synchronizing information, a luminance, or Y, signal is developed that closely resembles the monochrome signal developed by the black and white television camera. The luminance signal, which represents the luminance or brightness portion and much of a detail of an image, is used to replace the monochrome signal of the black and white system.
So that a color-television receiver will be able to reconstruct the R, B and G signals, a chrominance signal is also developed and combined with the luminance signal. The chrominance signal may be developed by normalizing the levels of the R, B and G signals and combining specific portions thereof so as to develop two signals: an I signal which includes 60% of the R signal, 32% of the inverted B signal and 28% of the inverted G signal and a Q signal which includes 21% of the R signal, 31% of the B signal and 52% of the inverted G signal. After the I signal is filtered by a 1.5 megahertz low-pass filter and the Q signal is filtered by a 0.5 megahertz low-pass filter, the I and Q signals are used to quadrature modulate a chrominance-carrier signal to develop the chrominance signal.
More specifically, from a 3.579545 megahertz reference signal, a Q carrier signal is developed which lags the reference signal by 147.degree. and an I carrier signal is developed which lags the Q carrier signal by 90.degree.. Following filtering, the I signal is used to modulate the I carrier signal, as in a balanced modulator, to develop a double-sideband suppressed-carrier signal and the Q carrier signal is similarly modulated by the Q signal. Next, the two modulated signals are combined to develop the chrominance signal. Finally, the chrominance signal is combined with the luminance signal for transmission therewith. Also combined with the luminance signal is a burst of the (roughly) 3.58 megahertz reference signal following each of the horizontal-synchronizing pulses of the luminance signal.
Because of the proportions of the R, B and G signals comprising the I and Q signals, a chrominance signal is developed having specific characteristics. The I and Q signals are such that complementary colors (colors which if added produced neutral result) are equal in amplitude and opposite in phase. For this reason, when the R, B and G signals are equal, the I and Q signals and the chrominance signal disappear. Further, characteristics of the chrominance signal are such that the instantaneous phase of the chrominance signal with respect to the reference signal, and the reference signal bursts, directly represents the hue of the respective portion of the image and the instantaneous amplitude of the chrominance signal indirectly represents the saturation of the color.
The chrominance signal also represents the sum of an R-Y signal modulating a carrier signal which lags the (roughly) 3.58 megahertz reference signal by 90.degree. and a B-Y signal modulating a carrier signal which lags the R-Y carrier signal by 90.degree..
It is important to note that the chrominance-carrier frequency, as specified by the NTSC, is 455 times the half-horizontal frequency. Additionally, because of the band width of the R-Y and B-Y signals and the modulation process, the chrominance signal contains sideband energy which extends below the chrominance-carrier frequency (roughly) 1.5 megahertz to (roughly) 2.1 megahertz and above the chrominance-carrier frequency (roughly) 0.5 megahertz to (roughly) 4.1 megahertz.
The NTSC standard color-television system and similar systems, such as the Phase-Alternation-Line, PAL, system, are particularly suited to developing, for transmission, black and white compatible color-television signals. Not only is the chrominance-carrier frequency sufficiently removed from the aural carrier so as to minimize beat patterns, but the chrominance-carrier frequency is sufficiently high so as to permit inexpensive color television receivers to easily separate the luminance and chrominance portions of the standard color-television signal, as by the use of a low-pass and a high-pass filter, respectively. Since the chrominance-carrier frequency is above the normal band width of inexpensive black and white television receivers, the chrominance portion of the broadcast signal causes minimal disruption thereto. Because the chrominance-carrier frequency is an odd multiple of the half-horizontal frequency, the chrominance portion of the signal, as displayed on the picture tube of a television receiver, alternates in phase on a line-to-line basis causing any residual portion thereof to cancel. Also, the TV masks employed in color television receivers filter any remaining chrominance portion of the signal.
Since the chrominance carrier frequency is an odd multiple of the half-horizontal frequency, the spectrum of the chrominance portion of the standard color-television signal tends to fall within holes of the spectrum produced by the luminance portion thereof. This relationship is important in that the luminance and chrominance portions of the signal may be readily separated using comb-type filters.
The NTSC standard color television system, and similar systems, are not particularly suited to developing color-television signals for recordation upon magnetic tape. As a result, a number of different systems have been developed for color television recording. One such system is exemplified by recorders of the type which are designated VR2000 by the Ampex Corporation. Each of these recorders employs a carrier-signal-generating oscillator that is frequency modulated responsive to the amplitude of the combined luminance and chrominance portions of the standard color-television signal, to develop a modulated signal suitable for recordation upon a length of magnetic recording tape. The modulated signal is used to drive four magnetic recording heads mounted in quadrature on the periphery of a two inch diameter rotating drum. The magnetic tape is drawn across the drum along a path parallel to the axis thereof such that the heads record the modulated signal upon a series of tracks which traverse the tape at an oblique angle with respect to the axis thereof.
When the carrier signal is modulated by the combined luminance and chrominance portions of the standard color-television signal, sideband energy is produced which extends above and below the carrier frequency in proportion to the band width of the combined signal. In order to prevent appreciable energy from extending below the carrier frequency into the upper spectrum of the combined signal to be folded back developing Moire type distortion, a relatively high carrier frequency is employed. The free-running carrier frequency and the depth of carrier modulation employed by the above-mentioned recorders are such that the frequency of the modulated signal is 7.06 megahertz responsive to sync-tip levels of the combined signal, 7.9 megahertz responsive to blanking levels of the signal and 10 megahertz responsive to reference white levels of the signal.
Unfortunately, the relatively high carrier frequency employed by the above-mentioned recorders requires the use of a relatively high tape speed and relatively complex circuitry to faithfully record and reproduce the modulated signal and thus the combined luminance and chrominance portions of the standard color-television signal. For applications in which the full band width of the luminance portion of the standard color-television signal need not be preserved, a system called color-under has been developed. Recorders employing the color-under system each develop a signal suitable to recordation upon a length of magnetic recording tape by frequency modulating a relatively lower frequency carrier-signal generating oscillator responsive to a signal derived by low-pass filtering the color signal (or a monochrome black and white signal).
By reducing the band width of the signal used to modulate the carrier, the spectrum of the modulated signal is also reduced permitting the use of a lower carrier frequency and lower tape speed without incuring Moire interference. Unfortunately, the reduced modulation band width is insufficient to process the normal chrominance portion of the standard color-television signal. For this reason, recorders employing the color-under system process separately the chrominance portion of the signal. First, color-under type recorders filter the color signal to recover the chrominance signal. Next, the chrominance signal is mixed with a higher-frequency local-oscillator signal to heterodyne the chrominance signal to a lower frequency. Finally, when the recorder ascertains that a color signal is being processed, as evidenced by the presence of reference-signal bursts, the lowered-frequency chrominance signal is combined, at a reduced level (reduced typically 6 db) with the frequency-modulated signal for recordation upon the tape.
A type of video recorder employing the color-under system is one which is designated U-matic by the Sony Corporation. U-matic recorders employ a carrier oscillator the free-running frequency and depth of modulation of which are such that a signal of 3.9 megahertz is developed responsive to sync-tip levels of the luminance portion of the standard color-television signal and a 5.4 megahertz signal is developed responsive to reference white levels thereof. Additionally, U-matic recorders heterodyne down for separate processing the chrominance portion of the signal to a carrier frequency of 688 kilohertz.
The modulated signal is used to drive a cylindrically-shaped scanner around which a length of 3/4 inch wide magnetic recording tape is wound in helical fashion. The scanner is comprised of a pair of coaxially-disposed drum-shaped portions including a rigidly mounted lower portion which guides the tape and an upper rotating portion which carries a pair of diagonally-opposed heads. The tape is drawn around the scanner such that the heads record the combined signal upon a series of tracks which traverse the tape at an acute angle with respect to the axis thereof. To prevent cross-talk, the tracks are separated by a series of guard bands.
The signal band width used to frequency modulate the carrier is further restricted by color-under recorders of the type known as Beta and VHS. VHS type recorders develop a modulated signal of 3.4 megahertz responsive to sync-tip levels of the filtered standard color television signal and 4.4 megahertz responsive to reference white portions thereof. These recorders heterodyne down the chrominance signal to a 629 kilohertz carrier frequency after filtering the chrominance signal to a band width of approximately 1 megahertz. To further conserve recording tape, VHS and Beta type video recorders do not use guard bands to reduce cross-talk. Rather, these recorders employ scanners in which the heads are suitably cocked.
There are a number of disadvantages associated with the separate processing of the chrominance portion of standard color-television signals as implemented in video recordings employing the color-under system. The signal-to-noise ratio of the chrominance portion of the signal is degradated, typically 12 db. A 6 db penalty is incurred because the chrominance portion of the signal is recorded as a double-sideband suppressed-carrier signal rather than as a frequency-modulated signal. Further, the chrominance portion of the signal is recorded at a level typically 6 db lower level than the level of the luminance-modulated signal.
By employing two different carriers at widely separated wave lengths for carrying the luminance and chrominance information, time-base instabilities are introduced by the recording process which are not coherent. Any errors are further multiplied when the reproduced chrominance signal is heterodyned back to the original chrominance-carrier frequency. As such, the chrominance-carrier frequency is no longer an odd multiple of the half-horizontal frequency. Therefore, it is no longer possible to separate the chrominance and luminance portions of the color signal using comb-type filters. Additionally, the chrominance signal no longer cancels on a line-to-line basis on a television display. Further, direct time-based correction (where a time-base corrector is locked to the reference-signal bursts) is also no longer possible. In addition, because the chrominance signal is heterodyned down to a relatively low carrier frequency, the band width of the chrominance information is restricted.
Although the use of cocked heads in VHS and beta type recorders is effective in reducing cross-talk between tracks for the modulated-luminance signal, the use of cocked heads is much less effective in reducing cross-talk at the wave lengths employed for the heterodyned-chrominance signal.